The so-called island is when the power grid accident happened or power paused for reparation, the user end of distributed generation system did not cut from the connected power grid, resulting in distribution generation and forming a self-contained power supply system with surrounding loads. The island includes planed island and non-planned island. In the planed island, the power of distributed generation can still continue to run after the main network outage, and supply power for a part of important loads. While in the planed island power supply must be stopped to the loads after the main network power off. Islanding detection is one of the key technical to solve the “grid-connection difficulty” of distributed generation. the accuracy of detection is the prerequisite of smooth handover between distributed generation grid-connected and islanded operation, is the basic condition to carry out the microgrid energy management system.
At present, the islanding detection method mainly includes local detection method based on local electrical quantities (these electrical quantities including frequency, voltage, harmonic, impedance and so on) and the remote detection method based on remote communication technology. The local detection is divided into passive detection and active detection methods. Passive detection method has inevitable detection zone in distributed generation system while the active detection method has reliability and interference problems. Detection cost will be pulled highly by making use of both methods synthetically. The remote detection method must real-time monitor switch status of the grid, tripping signal. this method relies on the support of communication network. The construction cost of communication network is a key obstacle. Moreover, another difficulty of the remote detection method is the random changes of distribution network topology.
With construction of the smart grid, there will be large distributed power demand in the distribution network. In the future, the distributed generation and microgrids will be applied popularly in china.